This invention relates to transmission and reception of video pictures and, more particularly, to the multiplexed transmission of two television pictures on the same link on a field-sharing basis and reception and reconstruction of the two pictures.
Because there is a limited number of radio frequency channels available, or because a given radio frequency channel may be available for only a limited period of time, there is a need for a system capable of transmitting more than one television picture over a single channel. For example, because of the limited number of video channels available in the satellite communication system in current use, any given user has a channel available to him for a few minutes a day and the cost per minute for use of the channel is significant. The growing appetite of the public for news, from both home and abroad, coupled with the attendant requirement to utilize satellite communication for timely foreign news coverage and its limited availability and high cost, has created a need for more effectively utilizing the limited transmission time available. If, for example, a domestic broadcaster in New York has ten minutes allotted to it on the link from London to New York, the pictorial news gathered in and about London in a given day must be edited at the sending end to present short segments of several newsworthy events and pre-taped to produce a program of precisely ten minutes duration for transmission over a single satellite channel during the allotted ten minutes. At the receiving end, the receiving material must again be edited to prepare for broadcast one or more segments of the received picture material. The pressure of editing would be somewhat relieved, and more news material could be transmitted in a given time interval, if more than one television picture could be transmitted on a common channel, or stated another way, the cost per minute of transmission of usable picture information could be reduced if more than one television picture could be multiplexed on a common channel.
Also, substantial savings could be realized in the archival storage of television programs if one were able to store, for example on video tape, only one field of each frame of a television program and to reconstruct the picture in a manner acceptable for later viewing. The current practice is to record the entire program on two-inch video tape, the cost of which is substantial and the bulk of which is such as to create a serious and expensive storage problem. By recording but one field of each frame the cost of the required video tape, and the rental cost for physically storing a given program could be cut in half.
Another area of application in which it would be desirable to transmit more than one television picture on a given channel is in the field of educational television. In many parts of the United States geographically separated institutions are tied together by microwave relay systems which enable transmission, for example, of a lecture given at one university to several other universities in the system. Such consortia of universities have regular programming sequences, but usually being limited to but one microwave channel, only one program at a time can be sent down a link. Clearly, the effectiveness of such systems would be enhanced if more than one video program could be transmitted on the same channel at the same time.
Techniques are known, from U.S. Pat. Nos. 3,725,571 and 3,745,242, for example, whereby a plurality of monochrome video pictures can be transmitted over a single television channel, on a line-sharing basis. The multiplex video transmission system described in these patents includes means for sending a plurality of n separate pictures wherein every nth line of each of the pictures is selected for transmission beginning at a different line and wherein reception of the selected one of the pictures is accomplished by selecting from the plurality of lines transmitted every nth line commencing at the preselected line. The selected line is delayed by up to n-1 delay lines capable of delaying the signal by n-1 horizontal lines and then recombined with the undelayed line so that a complete picture is provided for reproduction.
The line multiplexing technique described in these patents has several shortcomings which make it unacceptable for encoded color television. First, because much of the program material it is desired to transmit will be available at scanning standards other than NTSC, e.g., PAL or SECAM, it is necessary to preserve picture correlation among several successive lines in every field. To overcome this shortcoming would require use of two standards converters to permit multiplexing to be accomplished following conversion; since standards converters cost approximately half a million dollars, this solution would be prohibitively, and needlessly, expensive.
Secondly, line multiplexing produces a coarser structure than does field multiplexing on diagonal lines in a picture since the "pitch" of field lines is twice as great as that of the frame lines, even when some form of line interpolation is used. In this connection, U.S. Pat. No. 3,745,242 describes a technique of line interpolation wherein the line earlier is added to the present line in equal parts. The interpolation is done at the transmitter, which not only ruins the horizontal resolution of the entire transmission, but is applied to the delayed lines of both (assuming n= 2) of the programs being transmitted.
In the system of U.S. Pat. No. 3,745,242, the problem of picture "crawling" on diagonals (similar, but not the same as "NTSC crawl") is claimed to be eliminated by synchronizing the transmitter and receiver to reset after each frame so that the first picture is always transmitted on the odd lines of each frame and the second picture on the even. The requirement in a color television system for preserving continuity of color subcarrier would be seriously frustrated by resetting the line multiplex sequence every frame. This would demand a reversal of chrominance phase every frame and a further reversal of the process of the chrominance inversion that is an essential part of the line-repeating technique, which, in turn, would require some form of chrominance indexing of the television frames.
Television transmission systems are also known, from U.S. Pat. No. 3,586,767, which transmit two classes of information interleaved over the same channel. This prior system includes a television transmitter, a television relay unit and at least one receiver. The television transmitter includes means for generating fields of a first class of information and also means for producing fields of a second class of information. In general, the system is operative to substitute for one of the fields of a frame of a conventional commercial television program (general viewer information) a field of information intended for specific viewers (specific viewer information). The specific viewers could be, for example, students in a nationwide educational program, or researchers subscribing to a centralized information retrieval network. The fields are sequentially transmitted with at least one field of the second class of information inserted between two fields of the first class of information. There is also transmitted an indicator signal related to the time of occurrence of the field of the specific viewer information. The relay unit includes means for receiving the sequentially transmitted fields of information as well as control means which receive and are operative in response to the indicator signals for generating control signals. A retransmitting means in the relay unit controllably retransmits some of the received fields of information by utilizing a reconstructing means responsive to the control signals. The reconstructing means substitutes for the field of the specific viewer information one of the adjacent fields of the general viewer information so that the retransmitting means only retransmits fields of the general viewer information to the receiver.
In recovering the general viewer information the system substitutes in the field interval occupied by a field of specific viewer information the next preceding general viewer information field interlaced with itself by utilizing a delay unit designed to introduce a delay of at least one field. The patent suggests that the delay unit may be a magnetic disc provided with a recording head, a reading head located a field time downstream of the recording head, and an erase head located downstream of the reading head, with the disc rotated at a speed such that the writing head and recording head are circumferentially displaced along the track of the disc so that information entering the writing head leaves the reading head one field time later. The operativeness in color television of the described technique for interlacing the preceding field with itself in the "missing" field interval is questionable because with a one field delay, the interlaced lines would not be in geometric alignment; for proper alignment in the 525-line NTSC system the delay must be an integral number of lines, usually 263 lines in known slow motion and stop action applications, and in this event, it is further necessary to invert the chrominance signal of the delayed field in order that the chrominance not be cancelled out in the interlaced picture.
Essential to the recovery of the special viewer information is the provision of an indicator signal preceding the field in which the special viewer information is substituted for conditioning the special information decoding device. The nature of the indicator signal is not described except that it may be included in the vertical blanking signal of the field, or, may be a characteristic tone in the audio portion of the signal now being transmitted. The patent suggests that the density of fields of the specific viewer information interplexed or inserted between frames of the general viewer information can range from, say, one out of one hundred or more to the case wherein the fields alternate with general and specific classes of information, or that the alternating fields could comprise two different television programs. However, the patent does not disclose how these extensions of the described system could be accomplished, particularly since the system requires an indicator signal preceding the field in which substitution is made. Obviously, if an indicator signal were inserted in each and every field, it would lose its significance as an indicator.
Another system for transmitting two classes of video information on a common channel is described in U.S. Pat. No. 3,637,926 and includes a television transmitter including a source of fields of general viewer information and a source of fields of special viewer information. Included in the transmitter are means for superimposing a portion (e.g., seven to ten percent of full white level) of the field of special viewer information of a field of general viewer information to create a superimposed field, the patent stating that superposition of this portion of one field on another can be accommodated in most television programs without affecting the quality of the general viewer information. The fields of general viewer information are transmitted with the superimposed field or fields interposed therebetween, and at the receiver means are provided for separating out from the superimposed field or fields the field of special viewer information and for displaying the same. The separating means is conditioned by a first characteristic signal or indicator present at the start of the field containing a field of general viewer information on which is superimposed a field of special viewer information, and at the end of this received field a second characteristic signal which precedes the field containing the other field of the frame of general viewer information on which is superimposed the inverse of the field of special viewer information, causes transmission of the special viewer information signal to a suitable display. Thus, as in the system described immediately above, indicator signals must be included in the transmission from the transmitter to enable separation of the superimposed special viewer information from the general viewer information. The receiver includes a field store, such as the video disc delay unit described above, capable of storing one field for performing a video substraction to obtain an interlaced display of the special viewer information.
It is evident from the foregoing brief description of the relevant art of which applicants are aware that known systems do not contemplate and/or are incapable of transmitting two commercial color television programs over a single communications link, particularly when the program material to be transmitted is to be subsequently subjected to standards conversion from one color television standard to another. Accordingly, it is a primary object of the present invention to provide a relatively simple and inexpensive system for transmitting two color television programs over a common channel and reconstructing the two color programs at the receiver with a picture quality acceptable for broadcast and general viewing.